This invention relates to photoflash lamps and, more particularly, to flashlamps of the type containing a primer bridge, or the like, ignited by a high voltage pulse.
Such flashlamps typically comprise a tubular glass envelope constricted and tipped off at one end and closed at the other end by a press seal. A pair of lead-in wires pass through the glass press and terminate in an ignition structure including a glass bead, one or more glass sleeves, or a glass reservoir of some type. A mass of primer material contained on the bead, sleeve or reservoir bridges across and contacts the ends of the lead-in wires. Also disposed within the lamp envelope is a quantity of filamentary metallic combustible, such as shredded zirconium or hafnium foil, and a combustion supporting gas, such as oxygen, at an initial fill pressure of several atmospheres.
Lamp functioning is initiated by application of a high voltage pulse (e.g., several hundred to several thousand volts, for example, from a piezoelectric crystal) across the lamp lead-in wires. The mass of primer within the lamp then breaks down electrically and ignites; its deflagration, in turn, ignites the shredded combustible which burns actinically.
A considerable amount of prior development effort has been carried out in the area of ignition structures and primer materials for high voltage flashlamps. For example a copending application, now issued as U.S. Pat. No. 4,059,388 and assigned to the present assignee, describes a primer material comprising a mixture of combustible metal powder, one or more metal oxides, and a binding agent, but which is free of oxidizer salts. In this instance, the metal oxides function as an oxygen donor. Another copending application, now issued as U.S. Pat. No. 4,059,389 and assigned to the present assignee, describes a beadless ignition structure comprising a pair of spaced-apart lead-in wires with spherically shaped terminations, a glass frit coating over the lead-in wires, and a coating of primer material over the frit-coated terminations. The primer may bridge the wire terminations or comprise separate spaced apart coatings on the respective terminations, with the filamentary combustible being in contact with both terminations to provide a conducting path therebetween. A primer composition is described which comprises about 99.0 percent by weight of zirconium powder and 1.0 percent by weight cellulose nitrate on a dried basis.
Improved primer materials for high voltage type flashlamps are also described in U.S. Pat. Nos. 3,972,673 and 3,969,067. The primer material of the former comprises a solid mixture of combustible fuel and an oxidizer for the fuel, such as an alkali metal chlorate or perchlorate, and further contains a combustion supporting oxide of the type which is converted to a lower oxide upon combustion of the mixture. More particularly, the patent indicates that certain metal oxide additives in this solid primer mixture promote a more complete combustion of the primer fuel. It is hypothesized that the additive is partially reduced through chemical reaction taking place when the lamp is flashed to provide a source of oxygen which is readily available for combustion of the primer fuel by reason of the oxygen being generated in the solid mixture. The specific combustion-supporting oxides indicated as suitable for this application comprise Co.sub.3 O.sub.4, BaCrO.sub.4, Fe.sub.2 O.sub.3, and the higher oxides of nickel. A preferred primer material composition is given as comprising a solid mixture, in percentages by weight of 46.1 percent zirconium, 14.5 percent sodium chlorate, 31.7 percent Co.sub.3 O.sub.4, and 7.7 percent BaCrO.sub.4, and further containing between 1-5 percent of a water soluble polymer binder such as polyvinyl alcohol or polyvinyl pyrrolidone.
The second patent mentioned above, namely, U.S. Pat. No. 3,969,067, describes an improvement over the primer material discussed above in that the composition further includes an alumina gel additive in an amount from about 0.25-2.0 percent by weight of the solid mixture. The patent indicates that this additive modifies the operation of the primer material to promote less sensitivity to premature accidental ignition from ambient electrostatic charges without requiring an increase in the maximum energy provide by the firing pulse.
Previous primers we have employed for high voltage flashlamps have shown a behavior and sensitivity that is quite strongly influenced by the nature and idiosyncrasies of the particular lot of zirconium powder used. Upon studying the behavior of different zirconium lots in primers for high voltage flashlamps and attempting to correlate behavior with the known variables of the zirconium powder (e.g., particle size, purity, percent weight gain upon ignition, burning rate, and the levels of many specific chemical impurities present such as H, O, Fe, Mg, Si, N, Al, Ca), no clear relationship was found that would account for the observed variations in performance. For example, two different lots of zirconium powder manufactured from the identical raw materials and which are analytically similar may give vastly different levels of reliability when they are used in primer and fabricated into high voltage flashlamps. For reasons that are indeterminate, the zirconium powder appears to vary in ignitability.
In the earlier filament type flahslamps, differences in zirconium powder lots were recognized but the major effect that showed up had to do with variations in lamp output peak time. The ignition means in such lamps, e.g., mere heating of the primer in contact with a hot filament until ignition occurs, is sufficiently non-critical that actual primer ignition failure, rarely, if ever, was observed.
In contrast, the primer in high voltage flashlamps must function positively upon passage of a few microjoules of electrical energy in a pulse that lasts only for microseconds. The zirconium powder used in such primers must ignite reliably and sustain ignition upon such feeble and momentary heating. Consequently, the apparently inevitable differences that occur in zirconium powder from one manufactured lot to another are no longer a nuisance factor but rather can mean the difference between correct functioning and actual lamp failure.
It is obvious in such a situation that selection of zirconium lots for use in high voltage primer is a possible solution to the dilemma. This in fact has been done, so that only the best and most readily ignitable zirconium powder is used in high voltage flashlamps. In spite of such lot selection, however, slight differences in sensitivity and behavior are still detectable from one lot to another. This appears to be especially true in the safe, oxidizer-free primers that are favored for manufacturing use and which have been disclosed in the aforementioned U.S. Pat. No. 4,059,389.